1. Field
Percutaneous coronary intervention devices.
2. Description of the Related Art
Medical catheters generally include elongated tube-like members that may be inserted into the body, either percutaneously or via a body orifice, for a wide variety of diagnostic and interventional purposes. Such catheters are particularly useful with regard to certain cardiovascular applications where an object is to deliver a treatment or instrument to a region of interest in a blood vessel (e.g., artery or vein) to modify (e.g., treat) a remote lesion.
Percutaneous Transluminal Coronary Angioplasty (PTCA or balloon angioplasty) and stenting are the predominant treatment for coronary vessel stenosis. In PTCA or stenting, a catheter is inserted into the cardiovascular system via, for example, a femoral artery. A pre-shaped guiding catheter is positioned in a coronary artery, and a dilatation catheter having a distensible balloon portion with or without a stent is advanced through the guiding catheter into the branches of the coronary artery until the balloon portion traverses or crosses a stenotic lesion. The balloon portion is then inflated with a medium to compress the lesion in a direction generally perpendicular to the wall of the artery, thus dilating the lumen of the artery. If a stent is also delivered with the balloon, it remains in the blood vessel and acts as scaffolding to hold the blood vessel open.
The position of a catheter in a vessel can be monitored using MRI techniques. Briefly, MRI is an imaging technique primarily used in medical settings to produce high-quality images of the internal human body. MRI is based on principles of Nuclear Magnetic Resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules.
Generally, the human body consists primarily of fat and water. Fat and water have many hydrogen atoms that make the human body approximately 63 percent hydrogen atoms. The nucleus of a hydrogen atom is comprised of a single proton. A property called “spin” is possessed by a single proton in a hydrogen atom. Spin can be thought of as a small magnetic field that causes the nucleus to produce an NMR signal.
During magnetic resonance imaging, a MRI system generates a strong magnetic field. When a target object (containing water molecules or other hydrogenous compounds) is positioned in the field, the field aligns magnetic dipoles of the hydrogen nuclei within the target object (and other hydrogen atoms). The magnetic field strength required to so align the magnetic dipoles is typically on the order of one Tesla, but field strengths significantly higher and lower than one Tesla are also used in various applications of MRI. The magnetic field imparts a resonant frequency to the nuclei that is proportional to the field strength. Once aligned by the magnetic field, the magnetic dipoles can be rotated out of alignment by application of radio frequency (RF) energy at the resonant frequency of the system. Electromagnetic radiation is subsequently emitted by the resonating magnetic dipoles (i.e., the protons spinning at their resonance frequency) as they return to alignment with the field. Imaging occurs as a result of detecting such radiation emitted from each of many different regions within the target.
Signal transmission and reception are produced through use of a radio frequency (RF) transmitter connected to a transmitting coil or antenna within the imaging unit (an “MR scanner”) and a RF receiver connected to a “receiver coil” also located in the imaging unit. The receiving coil is positioned as close to the object as possible for maximum imaging sensitivity. The patient or object is often surrounded by a body coil that may serve both as transmitting and receiving antennae. Alternatively, the body coil can be used as a transmitting antenna only, and a separate surface coil is used as a receiving antenna. The surface coil can usually be placed closer to the tissues or object under examination than a single body coil. An RF oscillator generates radio waves of different frequencies. By controlling the magnetic field in a known way through a switching system that controls the current in the gradient coils, and by generating radio waves of a select frequency, the exact location at which the body of a patient or an object is imaged can be controlled. When the frequency of the RF signal is set for the exact value of the magnetic field, resonance occurs. Precession of the excited nuclear magnetic moment leads to induction of small currents in the receiving coil. The induced currents are detected to produce an output signal dependent upon the number of protons involved in the resonance and tissue-specific parameters. The output signal from the RF receiver is processed by a computer system to produce an image display of the tissue or the location of the RF receiver antenna.
While the ability to use MR imaging techniques to position a catheter to an area within the vasculature of a patient may be achieved with current technology, high resolution imaging of a targeted vascular site, such as stenosed vascular site is difficult to achieve. Recent research has indicated that in addition to a stenosed region of the vasculature, lesions referred to as thin-capped fibroatheromes (TCFAs) present a significant problem to cardiac function. TCFAs are lesions with large lipid pools, that are contained within the vessel wall by thin, fibrous caps. When a TCFA ruptures, a stenosed region may immediately form.
To diagnose TCFAs, current technology requires an imaging resolution of 50 to 100 microns. Current MR imaging techniques with a receiver coil placed external to a body of a patient do not provide adequate resolution to detect TCFAs. Obtaining a high resolution is complicated even in placing an RF antenna or coil in a blood vessel, because of blood vessel translation in response to cardiac pulsation and respiration; dilation/contraction of a blood vessel lumen and wall thickness in response to cardiac pulsation; and motion of the antenna or coil within a blood vessel due to hemodynamics.